The rotary movement of a rotary element of a machine, such as for example a drum of a washing machine, a centrifuge, or indeed a turbine, can lead to strong and harmful vibration being generated if the element does not present a distribution of mass that is balanced relative to its axis of rotation.
Such vibration is particularly pronounced for machines in which the drum rotating at high speed contains a free mass. This applies in particular to washing machines while spin drying.
In general manner, when beginning a stage of spin drying, the laundry contained in the drum of the washing machine is situated against the inside face of the cylindrical side wall of the drum, and it is distributed thereon in non-uniform manner. The laundry then represents a mass having its center of gravity that is off-center relative to the axis of rotation of the drum.
Because of its off-center position, the mass of laundry contained in the rotary drum exerts stresses on the drum which cause rotation of the drum to be accompanied by substantially circular movement of its axis. A drum is the observed to be rotating in unbalanced manner.
This unbalance, combined with high speeds of rotation (during spin drying) leads to high levels of vibration and of sound nuisance.
In order to attenuate this harmful vibration phenomenon, it is known to fit the drum of a washing machine with a balancing device seeking to avoid axial unbalance of said drum rotating at high speed.
One such device is known in particular from Document EP 1 634 986, comprising a balancing ring having mutually distinct channels that are filled in part with a balancing liquid.
Under the effect of the substantially circular movement of the axis of the drum, the balancing liquid in that device takes up a position in the channels of the balancing ring so as to be diametrically opposite the mass of laundry contained in the drum.
The assembly comprising the drum, the laundry it contains, the balancing ring, and the balancing liquid contained therein, thus becomes balanced relative to the axis of rotation of the drum, thereby substantially reducing the vibration generated by its rotation.
Such a balancing effect can thus be obtained for a home washing machine at speeds of rotation that may reach 200 revolutions per minute (rpm), for example.
For higher speeds of rotation, the centrifugal force that acts on the balancing liquid becomes preponderant compared with the inertial force associated with the substantially circular movement of the axis of the drum. That movement then no longer suffices to cause the balancing liquid to take up a position that is diametrically opposite the mass of laundry. In particular under the effect of the centrifugal force to which it is subjected, the balancing liquid contained in each channel becomes spread out over substantially the entire length of the channel. The unbalance due to the mass of laundry for spin drying is then no longer compensated by the balancing liquid, and the rotary motion of the drum is once more unbalanced and is accompanied by high levels of vibration.
Such a balancing device is thus of very little use in reducing the vibration generated by the rotation of a washing machine drum during spin drying, which rotation can typically reach a speed lying in the range 400 rpm to 1800 rpm.
In general manner, a balancing device such as that described in Document EP 1 634 986 is of little effect in balancing a rotary element that contains a free mass and that is subjected to high speeds of rotation, such as for example a drum of a centrifuge or of a washing machine.
Also known from Document WO 2010/029112 is a balancing device for a rotary drum that comprises a balancing ring with channels that are partially filled with a balancing substance having thixotropic properties. That substance is essentially solid at rest. It liquefies under the effect of the machine vibrating and then spreads out in the channel containing it. Document WO 2013/087722 describes such a balancing device, in which the sections of the channels can also be deformed.